Python: How to read multiple .NBT files and export to JSON?

I am a newbie when it comes to programming and i'm looking for a way to iterate through a directory filled with .NBT files and export them to JSON. I know i can loop thorugh a directory using os.listdir, but i have trouble reading the files one by one and deciding what steps to take in order to get it to a JSON format.

The actual assignment is to loop through a bunch of .NBT files to see which Minecraft crate is faced towards with direction.

This is what i have now:

import python_nbt.nbt as nbt
import os

for file in os.listdir("Directory to nbt files"):
    if file.endswith(".nbt"):
        filename = nbt.read_from_nbt_file(file)

From here i get a FileNotFoundError saying there is no such file or directory.

What am i doing wrong? And what must be done to continue?